1. Field of the Invention
The invention relates generally to marine seismic surveying. More specifically, the invention relates to a system and method for deploying seismic recorders.
2. Description of the Related Art
Seismic exploration is widely used to survey subterranean geological formations to determine the location of hydrocarbon formations in the earth. Reflection seismology is used to estimate the properties of the subsurface from reflected seismic waves. In reflection seismology, generated acoustic waves are propagated down through subterranean strata and reflect from acoustic impedance differences at the interfaces between various subterranean strata. The reflected seismic energy is recorded and processed to create an image of the subsurface structures. Because many commercially viable hydrocarbon formations are located beneath bodies of water, marine seismic methods have been developed.
In marine seismic survey systems it is typical to use geophone, hydrophones, or other seismic recorders (also referred to as sensors) to detect reflected seismic energy that is emitted from one or more seismic sources. These recorders are generally deployed in an array that may constitute one or more parallel lines. There are numerous ways seismic recorders can be deployed in a marine environment. In some instances, a streamer carrying seismic recorder sensors is towed near the surface behind a survey vessel. The streamer typically contains wiring to interconnect the sensors. Examples of these types of systems are found in, for example, U.S. Pat. Nos. 4,450,543 and 5,561,640. Because the survey cable is, in most instances, of unitary design and contains the seismic recorders wired together within the cable, one cannot change the spacing between the seismic recorders within the cable as may be desirable given a specific geological objective of the survey at hand. Also, because the reflected acoustic energy propagates through the water before being received by the seismic recorders in the streamer, noise significantly distorts the reflected energy. Also, because water has no shear strength, the aforementioned method is only capable of recording the vertical or pressure component of the full seismic wavefield.
In other instances, interconnected seismic recorders are placed directly on the bottom in a method typically known as Ocean Bottom Cable, or simply “OBC.” The seismic recorders in an OBC system are interconnected by reinforced cables that provide power and transmit data from the seismic recorders to a distal storage device. The rigid cables often allow good coupling with the bottom only along the major axis of the cable, significantly reducing the ability of the system, if equipped with three-dimensional geophones, to record the shear components of the full seismic wavefield. Additionally, one cannot readily change the spacing between the sensors within the cable as may be desirable given a specific geologic objective for a particular survey. Because this system relies on cables for power and telemetry, any damage to the cables or connectors, which is common in the marine environment, prevents the recording of data and contributes significant downtime and increased survey cost while the system is retrieved, repaired and redeployed. Also, the data and power cables contribute significant weight to the system which, combined with the reliability concerns, effectively prevents OBC systems from being deployed in deep water.
In other instances, autonomous nodal recorders are attached to a main cable by individual tethers, as disclosed in U.S. Pat. No. 6,024,344 to Buckley et al. The tethers interconnect a single attachment point on the recorder to a single attachment point on the cable. There are significant drawbacks with this type of arrangement. It is often necessary to remove the recorders from the main cable when the units are retrieved for charging, downloading and moving and then reattach the recorders immediately prior to re-deployment, which increase the handling effort and cost and also complicate the task of ensuring that the individual seismic recorders are deployed in the desired sequence. There is also a risk that the tethers could wrap around and get tangled in the main cable during deployment and potentially compromise data quality. If the main cable consists of a single length of cable and is damaged, the entire cable may have to be replaced at a significant financial cost and delay of operations. If the seismic recorders attach to the main cable at fixed attachment points, it is difficult to adjust the spacing between recorders, as may be necessary given the objective of a particular survey. Also, because the seismic recorders attach to the main cable only at a single point, the recorders will land on the ocean bottom at a completely random orientation relative to the other recorders and the survey geometry in general.